1. Field of the Invention
This invention relates to solar energy collector apparatus and, more particularly, to a solar energy collector apparatus and method whereby the shell structure for the solar collector is mounted on an equatorial mount for east-west tracking of the sun and the optical apparatus therein are movable so as to follow the seasonal variations of the sun without twisting or otherwise excessively bending the heat transfer conduits requiring flexible or rotatable couplings.
2. The Prior Art
Currently, the only inexhaustable source of energy available to mankind is solar energy. Solar energy or solar flux is customarily measured in langleys per minute, one langley being equivalent to one calorie of radiation energy per square centimeter. The intensity of the solar flux varies with geographical location, time of day, season, cloud cover, atmospheric dust, and the like, and this intensity varies between about zero and 1.5 calories per square centimeter per minute. Therefore, assuming a solar flux of one langley per minute, one square meter receives 10,000 calories per minute while a house roof, having 100 square meters, receives about 1,000,000 calories per minute. With an average of one langley per minute for 500 minutes per day (which is slightly more than 8 hours), the 100 square meter roof receives, in bright sunshine, about 500,000 kilocalories per day. This energy is the equivalent in thermal energy to burning about 14 gallons of gasoline. Therefore, solar energy represents a valuable, inexhaustable energy resource.
When an object such as a solar collector is exposed to solar radiation, its temperature rises until its heat losses become equal to its heat gains. The losses depend on the emission of radiation by the heated material, movement of the surrounding colder air, and thermal conductivity of the materials in contact with it. The gains depend upon the intensity of solar radiation and the absorptivity of solar radiation by its absorption surface. Customarily, solar energy is collected by two general techniques to produce higher temperatures: (1) by covering a receiving surface with a sunlight-transparent sheet of glass or plastic (flat plate collector), and (2) by focusing the solar radiation from a large area onto a receiver of small area (focusing collectors).
Flat plate collectors are usually stationary but should be repositioned every few days to follow the seasonal variations in the solar track. Flat plate collectors have the advantage of being generally cheaper to fabricate and also have the advantage in absorbing heat from diffuse solar radiation as well as the direct radiation by being able to operate on cloudy but bright days.
Focusing collectors can produce much higher temperatures although they can use direct radiation only and require turning throughout the day to follow the sun. Although focusing collectors are useful in obtaining higher temperatures from solar energy, (1) they usually cost more, (2) they need to be moved continuously to track the sun, and (3) they can use only direct solar radiation that is unscattered by clouds or haze. One common form of focusing collector is a parabolic mirror which has been used to obtain temperatures up to about 3500.degree. C. depending upon the optical perfection of the parabolic surface. Unfortunately, parabolic collectors are relatively expensive, require sophisticated mountings and the absorption surface is usually interposed between the sun and the parabolic reflector at a position adjacent the focal point of the parabolic curvature.
Another device for useful focusing solar energy in a focusing collector is the Fresnel lens. The Fresnel lens consists of nested grooves cut or otherwise formed in one face of a transparent material such as plastic. The sides of each successive groove is set in such a way that the light passing through each groove is refracted at a slightly different angle so as to converge on a common focal point or line. Such lenses have been pressed from rigid sheets of plastic material and are, therefore, relatively inexpensive while being effective to give a relatively sharp focus. A more detailed discussion on the use of a Fresnel lens in a solar concentrator can be found in "Large-Scale Fresnel Lens Solar Concentrator" Marshall Space Flight Center, Alabama; NASA Tech Briefs; Winter (1977) p. 461.
Since focusing collectors require tracking mechanism for tracking the sun, various types of tracking devices have been developed. Tracking of the sun in its east-west movement only is relatively simple since the sun moves at a rate of 15.degree. of arc every hour. This calculation is determined on the basis of the earth making one complete revolution of 360.degree. in a 24 hour period so that in one hour it moves 360.degree. divided by 24.degree. or 15.degree.. However, the annular motion of the earth relative to the sun causes the sun to appear to move in declination by about 47.degree.. This wide range from summer solstice to winter solstice is a major problem any focusing collector system must face. Thus, any fully tracking collector that is focused continuously on the position of the sun in the sky requires motion in two coordinates. While the exact coordinates in which the motion is made are not deemed important, one set of coordinates may be rendered redundant by using an equatorial mounting where one axis of rotation is supported parallel to the axis of rotation of the earth. The sun then appears to have no significant daily motion in the transverse coordinate (declination). Meanwhile, any other set of axes of motion requires two motions to track the daily motion of the sun. However, in order to accommodate seasonal variations, it is necessary to include within the equatorial mounting a mechanism for matching daily changes in the seasonal position of the sun. Thus, an equatorial mounting presents the more feasible mounting system for a tracking or focusing solar collector.
Additional information regarding solar collectors can be found in Applied Solar Energy, Aden B. Meinel and Marjorie P. Meinel, Addison-Wesley Publishing Company, Reading, Mass. (1976) Library of Congress Catalog Card No. 75-40904, and Direct Use of the Sun's Energy, Farrington Daniels, Ballantine Books, New York (1977) Library of Congress Catalog Card No. 64-20913.
Utilization of collected solar energy very often occurs at a location other than the center of focus for a focusing collector. The exception to this statement are those focusing collectors which are used primarily as photovoltaic power towers, solar cookers, etc. The technique for transferring solar energy from its collection site to its utilization site generally involves some form of fluid heat transfer medium. The fluid heat transfer medium is conducted through conduits to and from the solar energy absorption site. The heat transfer medium is heated by the thermal energy produced by the absorbed solar energy and carries the thermal energy to the utilization site where the thermal energy is either utilized directly or stored for subsequent use. Unfortunately, the combination of movable solar collectors and fluid heat transfer conduits presents difficulties with regard to fabricating solar collectors which will accommodate flexure or otherwise movement of the fluid heat transfer conduits.
In view of the foregoing, it would be an advantage in the art to provide improvements in solar collector apparatus and the method for collecting solar energy. It would also be an advancement in the art to provide a solar collector apparatus for tracking the sun, the apparatus including mechanism for accommodating the movement of the solar collector and the optical apparatus therein while minimizing excessive flexure or twisting of the fluid heat transfer conduits. Such an invention is disclosed and claimed herein.